Friday, October 16, 2009

A Black Hole Loses its Shirt

Physicists may have found a way to peel away a black hole's inescapable event horizon and look directly into the infinite weirdness that lies in its core. Many physicists including renowned cosmologist Stephen Hawking said this should be impossible, but Ted Jacobson of the University of Maryland thinks he might be on the right track to create the mythic "naked singularity."

What is a Black Hole

Black holes are gravity's greatest triumph. They're formed at the end of most massive stars' lives, after they've exhausted most of their nuclear fuel. The inward pull of gravity starts pulling all of the star's matter down into its center. A vicious cycle starts. As gravity shrinks the size of the star its density increases, amplifying the pull of gravity as it goes, further shrinking the size of the star …etc…etc. Ultimately, the star supernovas in a final blaze of glory, leaving behind a black hole.

Black holes are black because gravity around them pulls so strongly not even light can escape. When an object, say a spaceship, passes a black hole's point of no return, its "event horizon," the spaceship can only fall down towards its center with no hope of escape. So strong is this pull of gravity, even light travelling at 300,000 km per second, isn't moving fast enough to escape the dark maw of the black hole.

What is a Singularity

At the black hole's center is the singularity, a point of infinite density that contains all the black hole's immense mass in a volume smaller than an electron. The word "singularity" does not really do justice describing how truly weird these points are. An infinitesimally small point has, for all intents and purposes, zero volume. However black holes clearly have mass, rather a lot of it in fact. In order to find the density of singularities, we plug in the figures we know into the formula for density.

density = mass/volume

singularity density = (A lot of mass) / (0 volume)

The catch is, it is mathematically impossible to divide by zero. The laws of general relativity break down trying to describe singularity. In essence, physics fails.

How can a Singularity be Clothed

These singularities are at the heart of every black hole, but are so bizarre that many think there must be a "cosmic censorship principle" to keep them hidden behind event horizons, cut off from the sensible universe. Stephen Hawking has long been an advocate of keeping singularities clothed by black holes. In 1991, Hawking wagered there must be such a principle, betting $100 and a shirt ''embroidered with a suitable concessionary message" against Kip Thorne John Preskill. Hawking lost on a technicality in 1997, but still maintains that there must be a rigid cosmic censorship principle.

How can a Singularity be Naked

Jacobson however is looking for ways the break the censorship principle.

"Why should Einstein's equations have these modesty properties that clothe these singularities so that we can't see them," said Jacobson, lead author of the research, "It could be we just haven't looked in the right places for them yet."

Jacobson used computer models to simulate a collision between a spinning black hole and a rapidly rotating star. He found that if the speed and direction of rotation line up exactly right, the two combine to cancel out the force of gravity. The outward centripetal force of the spinning singularity matches gravity's inward pull, stripping away the event horizon exposing the singularity to the universe. Physicists have researched rotating black holes before, but Jacobson is the first to find a way to successfully expose its interior.

Jacobson's model is still incomplete however, and other physicists are looking to take it to the next level. Jacobson did not account for the complex effects of "back reaction" which causes the plummeting star to lose some of its energy to gravitational waves. Scott Hughes at MIT has a computer model that calculates gravitational waves, and plans to plug in Jacobson's scenario.

"I would love to be able to say definitively if the back reaction is enough to change the naked singularity outcome or not," Hughes said, adding also that he thought there was a good chance that the energy lost would prevent exposure of the black hole's center.

Depending on what Hughes finds, Jacobson says it could be possible to correct for any lost energy beforehand, disproving the cosmic censorship hypothesis. If not, then singularities may stay fully clothed forever.

"It's too bad because it would be nice to see a place where our theory [of relativity] breaks down," Jacobson said, "Cosmic censorship is probably the greatest fundamental question about general relativity as a theory."

1 comment:

  1. There is a new proposal for gravity on my blog.